This invention relates to a singly fed circularly polarized microstrip antenna.
A microstrip antenna has numerous unique, attractive features such as a low profile, light weight and conformable structure. Research of the microstrip antenna has been conducted for practical application to a broader field, such as the field of antenna on a flying object such as aircraft or satellites. Here, the microstrip antenna is put to practical application mainly as a circularly polarized microstrip antenna. The circularly polarized microstrip antenna is classified into a singly fed and a dual fed type, depending upon the number of feed points necessary to excite the circular polarized waves. The singly fed type is very useful, because it requires no external circular polarizer.
The metallic patch radiator of the conventional singly fed circularly polarized (SFCP) microstrip antenna has, for example, a nearly square configuration, a square configuration having a slot or cutout on the diagonal line thereof, a circular configuration having a slot or cutout on one diameter thereof, or an elliptical configuration. One feature common among all the configurations is that they are limited to linear symmetry configurations. The feed point of the patch radiator is located on two straight lines intersected at an angle of .+-.45.degree. with respect to the symmetrical axis and at an equidistant point of the symmetrical axis, i.e., located nearly on the two diagonal lines in the case of a rectangular configuration. In this case, the feed point, configuration and exciting frequency have been determined on a trial-and-error basis, requiring a lot of time and labor in the design of the antenna. A good circularly polarized wave was not excited with the conventional feed point. Furthermore, the exciting frequency is restricted to one frequency. The feature that the metallic path radiator has a linear symmetry configuration imposes a great restriction on the design of the antenna when the antenna is operated on a satellite in which weight and spatial room are restricted. Thus, the conventional singly fed circularly polarized microstrip antenna has involved various restrictions with respect to the configuration, exciting frequency, feed point etc.